The invention relates to a seal arrangement, and in particular to a seal arrangement capable of withstanding high pressures and suitable for use in the fuel supply system for a compression ignition internal combustion engine. The invention also relates to a method of manufacturing a seal arrangement.
In the fuel system for a compression ignition internal combustion engine, very high fuel pressures are commonly encountered. For example, in a fuel system of the common rail type, the fuel pressure within the common rail may be around 2000 bar. In order to achieve such high pressures, and in order to optimise efficiency, it is important to restrict leakage from the various parts of the fuel system, for example the fuel pump and the fuel injectors, to a very low level. In particular, at such high pressures the fuel pressure within bores formed in the components used in the fuel system may cause dilation of the bores and, as a result, the clearance between the walls of the bores and any members slidable within the bores may increase, thereby giving rise to increased fuel leakage.
European Patent Application No. 0 978 650 A describes a seal arrangement which aims to alleviate the problem of fuel leakage in a fuel system. The seal includes a support member having an outer periphery of circular form, the support member carrying a resilient seal member. The seal member has an outer periphery also of generally circular form and having a diameter which is greater than that of the outer periphery of the support member. When the seal member adopts a relaxed condition, the seal member and the support member are eccentric to one another. When the seal is compressed, the seal member and the support member are of substantially equal diameter and are substantially coaxial, the seal member being provided with slots which permit the seal member to be compressed so as to reduce the diameter of the outer periphery. In use, the seal may be introduced into the bore of a fuel pump within which a substantially fluid tight seal is to be formed, a piston being slidable within the bore and being received through the bore of the seal. The introduction of the seal into the bore requires compression of the seal member. When the seal is introduced into the bore, the resilience of the seal member causes the outer surface thereof to engage the wall of the bore in a substantially fluid tight manner. In use, the seal therefore adopts an internal position within the bore.
Internal seals of this type are only suitable for use in arrangements in which the piston is under compression within the bore. If the piston is not under compression within the bore, additional components may be required to maintain the seal in place. Furthermore, it can be desirable to coat the bore within which the piston is arranged with a wear resistant coating to reduce wear on the bore in the region of the seal. Such coatings can, however, be difficult to apply inside the bore.
It is one object of the invention to provide a seal arrangement suitable for use in a variety of fuel system applications whereby leakage of fuel can be restricted to an acceptable level, even when the fuel system is to be operated at high fuel pressures. It is a further object of the present invention to provide a seal arrangement which permits the aforementioned disadvantages to be mitigated or overcome.
According to a first aspect of the present invention there is provided a seal arrangement having a bore within which a component is to be received, in use, the seal arrangement comprising a seal member formed from a resilient material, the seal member having an inner periphery and an outer periphery and being provided with a fracture to define two ends of the seal member, the inner periphery of the seal member having, when in a relaxed condition, a smaller diameter than the diameter of the component such that, when the component is received through the bore of the seal arrangement, the two ends of the seal member are caused to part, the resilience of the seal member causing the inner periphery of the seal member to form a substantially fluid tight seal with the component, the seal arrangement further comprising a support member, upon which the seal member is mounted, the support member being of generally annular form and having inner and outer peripheries, the inner periphery of the support member being of generally circular form except for a region of irregularity.
The seal arrangement of the present invention provides the advantage that the inner periphery, as opposed to the outer periphery, of the seal member provides the sealing function. Thus, for the purpose of sealing a bore within which a piston member is slidable, the seal arrangement can be mounted externally of the bore, the inner periphery of the sealing member forming a substantially fluid tight seal with the outer surface of the piston member to substantially prevent fuel leakage from the bore. For conventional, internal bore mounted seals, the outer surface of the sealing member provides the sealing function. It is, however, often desirable to provide the surface of the bore with a wear resistant coating and this can be difficult to achieve. In the present invention, as the wear resistant coating can be applied to the outer surface of the piston member, the coating can be applied more easily. Furthermore, the seal arrangement of the present invention can be retained in position without the need for additional components, even if the piston member is not under compression.
Conveniently, upon parting, the two ends of the seal member define a clearance gap. The seal arrangement of the present invention provides the further advantage that, in use, as the outer surface of the piston member becomes worn, the clearance gap tends to close, thereby further reducing fuel leakage from the bore within which the piston member is slidable. A similar advantage is obtained if the bore of the seal arrangement becomes worn.
Conveniently, the diameter of the inner periphery of the support member forms a clearance fit with the component to be received through the bore of the seal arrangement, except for the region of irregularity.
The region of irregularity is conveniently provided by forming a relieved section on the inner periphery of the support member, the relieved section having a smaller radius of curvature than that of the generally circular section of the inner periphery of the support member so as to engage with a portion of the component, in use.
Preferably, the relieved section is substantially aligned with the clearance gap. In this way, any residual fuel leakage which may occur through the clearance gap is prevented.
The outer periphery of the support member may be of generally circular form. Conveniently, the outer periphery of the support member may be provided with an alignment feature which permits the region of irregularity to be aligned with the clearance gap upon assembly of the seal.
The outer periphery of the seal member may be of generally circular form. Alternatively, the outer periphery of the seal member may be shaped to include regions of relatively large diameter and regions of relatively small diameter.
The regions of relatively small diameter increase the flexibility of the seal member. This reduces the frictional loading on the outer surface of the component to be received through the bore of the seal. Furthermore, during manufacture, the regions of relatively large diameter provide a means of support for the seal member and also provide convenient points at which the seal member can be secured to the support member.
According to another aspect of the invention, there is provided a fuel pump comprising a piston member slidable within a bore, a surface associated with the piston being exposed to fuel pressure within a chamber, the piston member carrying a seal arrangement of the type described herein, the inner periphery of the seal member of the seal arrangement engaging the outer surface of the piston member to substantially prevent fuel from flowing between the bore and the chamber when the piston member moves within the bore.
According to a further aspect of the invention, there is provided a fuel injector comprising a piston member slidable within a bore, a surface associated with the piston being exposed to fuel pressure within a chamber, the piston member carrying a seal arrangement of the type described herein, the inner periphery of the seal member of the seal arrangement engaging the outer surface of the piston member to substantially prevent fuel from flowing between the bore and the chamber when the piston member moves within the bore.
It will be appreciated that reference to a flow of fuel between the bore and the chamber includes the flow of fuel from the bore to the chamber and vice versa, depending on orientation.
The piston member may comprise a first region of relatively large diameter and a second region of relatively small diameter, the first region being slidable within a first bore and the second region being slidable within a second bore, the first bore and the second bore being substantially co-axially aligned, the second bore being of larger diameter than the diameter of the second region so as to define a clearance between the second bore and the second region, the seal member being arranged to sealingly engage the outer surface of the second region of the piston member so as to substantially prevent fuel flow therepast.
The clearance between the second bore and the second region of the piston member enables any eccentricities between the first and second bores to be accommodated.
According to a further aspect of the present invention, there is provided a pressure regulating valve for controlling fuel pressure within a passage, the valve comprising the seal arrangement as described herein, the seal arrangement being in sealing engagement with the component received through the bore of the seal arrangement and the passage being defined, in part, by the component.
According to a further aspect of the invention, there is provided a method of manufacturing a seal arrangement as herein described comprising the steps of:
providing a seal member formed from a resilient material with a region susceptible to fracture,
applying a force to the seal member so as to controllably fracture the seal member at the region.
The region susceptible to fracture may be a stress concentration or an aperture.
Conveniently, the method may include the step of inserting a member through the bore of the seal arrangement or by applying a shear force to the seal member in the region susceptible to fracture in order to fracture the seal member. The member inserted through the bore to fracture the seal member may be the component to be received through the bore of the seal arrangement, in use, the component being provided with a chamfer.
The method may comprise the steps of;
providing a support member upon which the seal member is to be mounted,
securing the seal member to the support member,
forming a bore through the support member and the seal member, the bore being shaped such that an inner periphery of the support member is formed with a generally circular form except for an irregularity which is substantially aligned with the region of the seal member which is susceptible to fracture.
Preferably, the step of applying a force to the seal member to fracture the seal member is carried out after the support member has been secured to the seal member.